The Catacombs are located within Hogwarts. The entrance is in a tucked-away area of the seventh floor, hidden behind a suit of armor. It is said to only open for a member of the Halifax family, but others can open it with certain difficult dark magic or if they have a vial of a Halifax's blood.
The Catacombs frequently change. Anyone who enters is faced with a series of dangerous challenges. Those who reach the end will supposedly find something very valuable, usually stated to be treasure.
During the Septet's first year, Elliott Scott ventured into the catacombs. He reached the end, but found that the gold was only an illusion. He attempted to kill Lysander Halifax, who was acting through Jackie Halifax, but Lysander killed him first.